1. Field of the Invention
The present invention relates to an image sensor having a heterojunction bipolar transistor (HBT) and, more specifically, to an image sensor in which a complementary metal oxide semiconductor (CMOS) device and a silicon-germanium HBT (SiGe HBT) are mounted on a substrate using SiGe bipolar junction transistor CMOS (SiGe BiCMOS) technology, and a method of fabricating the same.
The image sensor according to the present invention can be more highly integrated than a conventional CMOS image sensor (CIS), and more highly sensitive to signals than when it is fabricated using typical bipolar technology because the amplification rate of signals becomes higher in a floating base reverse-operational mode. Further, the image sensor according to the present invention is excellent in a three-color balance characteristic since a surface SiGe or SiGe carbon (SiGeC) layer disposed on a photo diode (PD) shared with a base is a high-doped p-type layer with a small thickness.
2. Discussion of Related Art
Conventional image sensors may be generally classified as a low-cost CMOS image sensor (CIS) and a high-performance charge-coupled device (CCD) image sensor.
The CCD image sensor is widely used owing to good image quality even in a highly integrated pixel. However, since the CCD image sensor requires a high voltage of 10V or higher, a nanoscale CMOS circuit cannot be easily applied to the CCD image sensor, so it is difficult to make the CCD image sensor highly integrated. Also, fabrication of the CCD image sensor is incompatible with that of typical CMOS devices. Thus, an image sensor module is made bulky because it needs to include at least two chips. Further, the CCD image sensor consumes at least ten times as much power as the CIS due to the high voltage, thus it is not suitable for portable data terminals.
On the other hand, the CIS includes a photo diode (PD) and a transistor in each image pixel like a typical CMOS device, and thus a conventional CMOS semiconductor fabrication process can be applied to fabrication of the CIS without making any changes. Therefore, as compared with the CCD image sensor requiring an additional chip with an image signal processor, the CIS can integrate an image signal processing circuit and an image sensing circuit in a block outside a pixel, operate at a low voltage, and be fabricated at low cost.
A typical CIS with a four-transistor pixel structure is illustrated in FIG. 1. The four-transistor pixel structure includes four transistors. Thus, a unit pixel is comprised of a PD, which is an optical sensor, and four NMOS transistors. Specifically, a transfer transistor serves to transfer photo-charges generated in the PD to a floating diffusion node region (FD), a reset transistor serves to discharge photo-charges stored in the FD or the PD in order to sense signals, a driving transistor serves as a source follower transistor, and a selection transistor is required for switching/addressing operations.
The PD and a capacitor (not shown), which are located thereto, constitute a receiving unit, and the transfer transistor transfers electrons generated by photons to the FD. In order to obtain a two-dimensional image, an electric potential is applied through a gate of the switch transistor to select one column. In particular, each pixel is biased by a current source (not shown), which operates the driving transistor and the selection transistor to read an electric potential at the diffusion node through an output node.
In the last five years, the development of the above-described CIS has progressed in earnest. At present, the CIS is being laboriously developed by Micron, Samsung Electronics, MagnaChip semiconductor, and so on. However, the CIS is being fabricated using CMOS technology in the 180-nm regime, but fabrication of sub-90-nm nanoscale CISs has not yet been attained.
In order to increase the area of the PD of a nanoscale CIS, a variety of methods for reducing the number of required transistors in a pixel and overcoming degradation of signals at low exposure have been proposed. One of the methods is to introduce a bipolar amplification type image sensor.
In the bipolar amplification type image sensor, an optical signal generated in a pn-junction-based PD may be amplified by a normal bipolar transistor or a reverse-operational bipolar transistor in which an emitter and a collector exchange functions using a floating base.
A method using the normal bipolar transistor is advantageous in a gain characteristic, but it has no effect on reducing the area of a pixel because the normal bipolar transistor only replaces a CMOS transistor of a CIS, for amplification use. In contrast, a method using the reverse-operational bipolar transistor can amplify the optical signal on its own, but its gain characteristic is poor. For this reason, the reverse-operational bipolar transistor needs an additional amplifier so that the area of a pixel cannot be reduced.